To overclock the Sapphire HD4870 Toxic, I started off by raising the GPU speed a little at a time and testing to make sure it was stable. If it was, I added some more to it until I could not run any stability tests. Once I hit the wall for the GPU speed, I used the same method for the memory until I hit the maxumim stable. The walls I hit were 845 on the core and 1066 on the memory. For a card that was already overclocked, the additional speed gained was very nice. Also while testing the card overclocked, the temperatures remained around 67C degrees with the fan speed controlled by the Catalyst Control Center, which is fairly cool for an HD4870 that is running even at stock speeds. By bumping up the fan speeds, the temperatures plummet down into the 50C range at 55C.